1. Field of the Invention
This invention relates generally to semiconductor processing and, more particularly, to metal silicides and methods for making metal silicides.
2. Description of the Related Art
Metal silicides are a commonly used material in semiconductor processing. As is well known, semiconductor processing is typically employed in the fabrication of integrated circuits, but such processing is also employed in a variety of other fields. For example, semiconductor processing techniques are often employed in the fabrication of flat panel displays using a wide variety of technologies and in the fabrication of microelectromechanical systems (“MEMS”). Metal silicides are commonly used, e.g., in integrated circuits, to form electrical contacts with low contact resistance (forming ohmic contacts). As a result, metal silicides having a low electrical resistance are desirable.
After a silicide is formed, the silicide is typically subjected to additional processing steps. For example, when a silicide is used to form source/drain contacts for a transistor, a dielectric, such as borophosphosilicate glass (“BPSG”), is deposited over the silicide to insulate the transistor and the source/drain contacts from other conductive elements. The BPSG is then annealed at a high temperature to cause it to reflow, to planarize the BPSG layer, thereby facilitating later processing steps.
It will be appreciated that as the sizes of electrical features decrease, changing process requirements have necessitated the development of new silicides. Nickel silicides (NiSi) have emerged as a possible candidate for use in fabricating electrical devices with very small feature sizes.
NiSi, however, has been discounted for use for many applications because of its instability at high temperatures, particularly those temperatures used in conjunction with the planarization of dielectrics, such as BPSG. While such planarization anneals are typically performed at temperatures of 700-950° C., NiSi is thermally unstable at such high temperatures. At higher temperatures, NiSi can lose its structural integrity. For example, nucleation or agglomeration can occur at these temperatures, causing voids and discontinuities. The resulting discontinuous silicide has an undesirably high electrical sheet resistance. In addition, spiking of the silicide can occur at high temperatures, causing the silicide to extend into a substrate and possibly causing an electrical short with other conductive features.
Variants of nickel silicides have also been developed and investigated. Such variants include carbon-doped nickel silicides. Ni-silicided carbon-doped Si films containing up to 1.3 atomic % carbon have been reported. Undesirably, however, carbon doping increases the resistance of the silicide, with increasing carbon-doping leading to increasing resistance, which runs counter to the desire for low resistance for many applications, including the formation of contacts. Moreover, the sheet resistance of the carbon-doped nickel silicide increases significantly as a function of temperature at temperatures greater than about 800° C. See, e.g., S. Mertens, The Electrochemical Society Meeting, Oct. 30, 2006, Cancun, Mexico. It will be appreciated that this increase in resistance coincides with the temperatures for typical BPSG anneals, which are typically performed at greater than about 700° C.
Accordingly, there is a need for silicides having good high temperature stability and good electrical resistance behavior after processing at high temperatures.